Compensation for digitally controlled oscillator apparatus and method

ABSTRACT

Automatic digital sensing and compensation of frequency drift caused by temperature, aging, and/or other effects may be provided by including a compensation capacitor array and a sensing logic. The sensing logic may be configured to detect a drift in a first control signal and to provide the compensation capacitor array with a second control signal. The second control signal is configured to cause an adjustment of capacitance in the compensation capacitor array based on the detected drift in the first control signal.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a national phase entry under 35 U.S.C. §371of International Application No. PCT/US2011/060068, filed Nov. 9, 2011,entitled “COMPENSATION FOR DIGITALLY CONTROLLED OSCILLATOR APPARATUS ANDMETHOD”, which designates, among the various States, the United Statesof America, and the entire contents and disclosures of which are herebyincorporated by reference in their entireties.

TECHNICAL FIELD

This disclosure relates generally to electronic circuits. Moreparticularly but not exclusively, the present disclosure relates toautomatic digital sensing and compensation of frequency drift.

BACKGROUND INFORMATION

In ring oscillators and inductance-capacitance voltage-controlledoscillators (LC-VCO), temperature drift is often solved by usingtemperature sensing and compensation circuits. For example, typical VCOimplementations may use a Proportional to Absolute Temperature (PTAT)circuit to counter temperature drift of oscillator cores. In anotherexample, a silicon chip with on-die temperature sensors may use atemperature sensor code as a mechanism to compensate for temperaturedrift. In the above approaches, temperature compensation is an open-loopmechanism, thereby requiring careful device characterization andpost-silicon trimming.

In contrast, a closed-loop approach might include detecting a controlvoltage of the oscillator. If the control voltage is too far from apreset operating point, an error signal can be fed back to adjustcommon-mode inputs of a varactor coupled to the LC-VCO, thereby forcingthe control voltage to the preset voltage. This implementation in theanalog domain, however, can be costly due to various reasons.

Another approach is to provide no compensation at all, in which casefrequency coverage range of a fine-tuning curve is simply extended. Inthe case of a digitally controlled oscillator (DCO), however, this maymean longer fine-tuning word length, thereby potentially incurringhigher logic power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments are described with referenceto the following figures, wherein like reference numerals refer to likeparts throughout the various views unless otherwise specified.

FIG. 1 illustrates an example of a digital phase locked loop including acompensation capacitor array in accordance with embodiments.

FIG. 2 illustrates a method performed by a sensing logic coupled to thecompensation capacitor array of FIG. 1, in accordance with embodiments.

FIG. 3 is a flow diagram further illustrating the method of FIG. 2.

FIG. 4 is a block diagram that illustrates an example computer systemsuitable to practice the disclosed compensation capacitor arrayaccording to methods of various embodiments.

DETAILED DESCRIPTION

Embodiments for a method and apparatus for automatic digital sensing andcompensation of frequency drift caused by temperature, aging, and/orother effects are described herein. In embodiments, a compensationcapacitor array coupled to a sensing logic in a digitally controlledoscillator (DCO) may be included in an all-digital phase locked loop.Although the embodiments are described in context of a phase lockedloop, other suitable circuit configurations are also contemplated. Inthe following description, numerous specific details are given toprovide a thorough understanding of embodiments. The embodiments can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring aspects of the embodiments.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearances of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments.

FIG. 1 illustrates a phase-locked loop (PLL) circuit 100 in accordancewith one embodiment. In the embodiment, PLL circuit 100 is anall-digital PLL that has an architecture that includes a frequency andphase detection circuit 104, frequency centering logic 106, and digitalloop filter logic 108, each coupled to a digitally controlled oscillator(DCO) 128. In various embodiments, DCO 128 may include aninductance-capacitance (LC) based DCO or ring-based DCO. In embodiments,DCO 128 may include a sensing logic 112 coupled to a respective firstand second capacitor array, such as for example, compensation capacitorarray 120 and fine-tuning capacitor array 122. DCO 128 also includes athird capacitor array such as a coarse-tuning capacitor array 124.

In embodiments, compensation capacitor array 120 may compensate fortemperature and aging effects on PLL circuit 100. In embodiments,compensation capacitor array 120 may include a number of m fine-tuningcapacitor arrays similar or identical to fine-tuning capacitor array122. Note that in embodiments, PLL circuit 100 may provide a referenceclock signal, F_(ref) 102, to frequency and phase detection circuit 104,which may in turn provide signals beginning along a respective upperpath 105 and lower path 107. In embodiments, upper path 105 may beincluded in a coarse-tuning path and lower path 107 included in afine-tuning as well as aging and/or temperature compensation path forre-centering a frequency of DCO 128. Thus, as can be seen from FIG. 1,frequency and phase detection circuit 104 may receive reference clocksignal F_(ref) 102 and a feedback signal F_(vco) 126 from DCO 128.Accordingly, frequency and phase detection circuit 104 may comparereference clock signal F_(ref) 102 to feedback signal F_(vco) 126 andoutput a signal along upper path 105 which includes a difference betweenfeedback signal F_(vco) 126 and a target frequency, F_(ref)*m, tofrequency centering logic 106. Target frequency, F_(ref)*m may beassociated with re-centering the frequency of DCO 128 to a centeroperating point of PLL circuit 100.

As noted above, compensation capacitor array 120 may include a number ofm fine-tuning capacitor arrays similar or identical to fine-tuningcapacitor array 122. Note that in embodiments, m may be programmable ata frequency setting 110 in FIG. 1. Accordingly, in embodiments, based atleast in part on a signal received from frequency and phase detectioncircuit 104, frequency centering logic 106 provides coarse-tuningcapacitor array 124 with a control signal or coarse control word (CCW)114. In embodiments, CCW 114 may be configured to control an enabling ordisabling of capacitors in coarse-tuning capacitor array 124 to bringfeedback signal F_(vco) 126 closer to target frequency F_(ref)*m. Invarious embodiments, frequency centering logic 106 may determine CCW 114through a linear search or a binary search algorithm. Note that inembodiments, CCW 114 may remain fixed after a calibration, unlessfrequency setting 110 is changed. In embodiments, coarse-tuningcapacitor array 124 may provide coarse frequency tuning by cancellingprocess-voltage-temperature variation during operation of PLL circuit100.

According to the embodiment shown, fine frequency tuning and temperatureand/or aging compensation may be accomplished along a flow includinglower path 107 of PLL circuit 100. In embodiments, frequency and phasedetection circuit 104 may compare a phase of feedback signal F_(vco) 126to a phase of reference clock signal F_(ref) 102. In embodiments, aphase difference may be included in a signal transmitted via digitalloop filter logic 108 to sensing logic 112. In embodiments, using thereceived signal, sensing logic 112 may determine a fine control word(“FCW”) 116 and a compensation control word (“comp. control word”) 118.Sensing logic 112 may then provide FCW 116 to fine-tuning capacitorarray 122 and comp. control word 118 to compensation capacitor array120. In embodiments, FCW 116 may be configured to allow fine-tuningcapacitor array 122 to correct a phase difference by enabling ordisabling a number of capacitors in fine-tuning capacitor array 122 toadjust capacitance.

Similarly, in embodiments, comp. control word 118 may be configured tocause an adjustment of capacitance in compensation capacitor array 120by enabling or disabling a number of capacitors in compensationcapacitor array 120. In embodiments, comp. control word 118 may be basedon a detected drift in FCW 116. In embodiments, if FCW 116 at time n(“FCW (n)”) has drifted away from a value or code associated with acenter operating point of PLL circuit 100 and stays beyond a threshold(“FCW_TH”) toward a negative or positive direction, one or morecapacitors in compensation capacitor array 120 may be enabled ordisabled so that a next FCW may be shifted back toward the centeroperating point. Thus, in embodiments, sensing logic 112 includes adrift sensing and decision-making circuit and may execute an algorithmsuch as for example, a method or algorithm 200 as shown in FIG. 2.

Thus, as shown in the embodiment of FIG. 2, at initialization 201, acurrent comp. control word (“CompCW” in FIG. 2) and a current finecontrol word (“FCW”) may be initialized to a code associated with acenter operating point, e.g., 0. Next, at 203, for the embodiment, afine control word at a time n, FCW(n) may be calculated by adding to aprevious FCW (“FCW(n−1)”), a difference between a current digital loopfilter output and a previous digital loop filter output from digitalloop filter logic 108. If at 205, determined by passing through a lowpass filter, an absolute value of FCW(n) is greater than a fine controlword threshold, FCW_TH, then at 207, CompCW(n) may be shifted in apositive or negative direction in a same direction as a sign of FCW(n)by at least one least significant bit (LSB) in the embodiment. Note thatthe low pass filter may process k previous samples of FCW(n). As notedpreviously, in an embodiment, to cover temperature and aging drift,compensation capacitor array 120 may include m identical fine-tuningcapacitor arrays. Therefore, in embodiments, a least significant bit ofCompCW, “CompCW_LSB,” may be equivalent to m*FCW_LSB. Thus, inembodiments, at 209, FCW(n) may be updated by shifting FCW(n) in anopposite direction of the sign of FCW(n) by m*FCW_LSB. Note that inembodiments, the low pass filter may suppress high-frequency noise.Thus, in embodiments, if FCW(n) remains beyond FCW_TH for a certainperiod of time, an adjustment to compensation capacitor array 120 may beneeded.

The method of FIG. 2 may also be illustrated in a simplified flowdiagram 300 of FIG. 3. Thus, beginning at 301, according to embodiments,a current FCW and CompCW may be initialized to 0. In the embodimentshown, at a next block 303, FCW (n) may be calculated according to anequation as shown in FIG. 2, such as, for example, FCW (n)=FCW(n−1)+digital_loop_filter_out (n)−digital_loop_filter_out (n−1). Theflow then may move to a next decision block 305. In the embodiment, if|FCW (n)|>FCW_TH, the flow diagram branches to blocks 307 and 309. Inblock 307, CompCW(n) may be computed and updated. Thus, CompCW(n) may beshifted in a positive or negative direction according to a sign ofFCW(n). Thus, in the embodiment, as shown in FIG. 2 at 207,CompCW(n)=CompCW(n−1)+sign(FCW(n))*CompCW_LSB. Accordingly, for theembodiment, at block 309, FCW(n) may be computed and updated by shiftingFCW(n) in an opposite direction of the sign of FCW(n), according to anequation such as FCW (n)=FCW(n)−sign(FCW(n))*m*FCW_LSB. In theembodiment, the flow from block 307 and 309 may then move to block 313where current time n then becomes previous time n. In the embodiment,the flow moves to a next sample and then loops again to block 303 tobegin an iteration to compute FCW and CompCW for a next time where(n−n+1).

Note that in the embodiment shown, if |FCW (n)| is not greater thanFCW_TH, then the answer at decision block 305 is No and the flow movesto block 311 where FCW (n) and CompCW (n) may remain static.Accordingly, FCW(n) may be set to a previous FCW, also referred to as,FCW(n−1). In embodiments, CompCW(n) may then also be set to previouscomp. control word, also referred to as CompCW(n−1). In the embodiment,the flow from block 311 may then move to block 313 for a next sample.

Thus, instead of relying on temperature detection using temperaturesensors, embodiments may take one or more of temperature drift and aginginto consideration by analyzing a fine control word for a fine-tuningcapacitor array or FCW 116 of FIG. 1. Thus, embodiments may provideautomatic digital sensing and compensation of frequency drift caused bytemperature, aging, and/or other effects. The drift in FCW 116 may bedetected and compared against a preset threshold such as a digital codeand based on a deviation from the present threshold, correspondingadjustment may be made to compensation capacitor array 120. Inembodiments, this implementation may occur entirely in a digital domainby adding a sensing logic 112 and compensation capacitor array 120 toPLL circuit 100. In embodiments, sensing logic 112 may be included indigital loop filter logic 108. Note that the method of FIG. 2 and FIG. 3describe only one example of an algorithm that may be used to implementembodiments and that other permutations of the algorithm are alsocontemplated. Further note that the methods of FIGS. 2 and 3, along withthe matching between compensation capacitor array 120 and fine-tuningcapacitor array 122 may ensure that significant disturbance to outputfrequency of feedback signal F_(vco) 126 (e.g., clock jitter) will notoccur.

Embodiments of automatic digital sensing and compensation of frequencydrift by compensation capacitor array 120 and sensing logic 112 may beused in a number of implementations and applications. FIG. 4 is a blockdiagram that illustrates an example computer system 400 suitable topractice the disclosed compensation capacitor array and sensing logic ofvarious embodiments.

As shown, computer system 400 may include a power supply unit 402, anumber of processors or processor cores 404, a memory 406 havingprocessor-readable and processor-executable instructions 408 storedtherein, a mass storage device 410 that may also store the instructions408, and a communication interface 412 including components 418. For thepurpose of this application, including the claims, the terms “processor”and “processor cores” may be considered synonymous, unless the contextclearly requires otherwise.

In various embodiments of the present disclosure, at least one of theprocessors 404 may include or be configured to control a detectioncircuit and a compensation capacitor array wherein the detection circuitmay detect a drift in a control word to adjust a second control word toadjust capacitance in the compensation capacitor array based on thedetected drift in the first control word.

The one or more mass storage devices 410 and/or the memory 406 maycomprise a tangible, non-transitory computer-readable storage device(such as a diskette, hard drive, compact disc read only memory (CDROM),hardware storage unit, and so forth). The computer system 400 may alsocomprise input/output devices 414 (such as a keyboard, display screen,cursor control, and so forth). In various embodiments, the communicationinterface 412 may include electronic component(s) 418 that may include acompensation capacitor array and sensing logic configured to compensatefor temperature and/or aging effects on a circuit and as described inaccordance with FIGS. 1-3. Such component(s) 418 may alternatively oradditionally be located elsewhere in the computer system 400, and maycomprise part or all of an integrated circuit.

The various elements of FIG. 4 may be coupled to each other via acommunication pathway 416, which may represent a multi-drop bus or apoint-to-point interconnect. In the case of multiple buses, they may bebridged by one or more bus bridges (not shown). Data may pass throughthe system bus 416 through the I/O devices 414, for example, between thecomponent(s) 418 and the processors 404.

The memory 406 and the mass storage device 410 may be employed to storea working copy and a permanent copy of the programming instructionsimplementing one or more operating systems, firmware modules or drivers,applications, and so forth, herein collectively denoted as 408. Thepermanent copy of the programming instructions may be placed intopermanent storage in the factory, or in the field, through, for example,a distribution medium (not shown), such as a compact disc (CD), orthrough the communication interface 412 (from a distribution server (notshown)).

The remaining constitution of the various elements of the computersystem 400 is known, and accordingly will not be further described indetail.

The above description of illustrated embodiments, including what isdescribed in the Abstract, is not intended to be exhaustive or to belimited to the precise forms disclosed. While specific embodiments andexamples are described herein for illustrative purposes, variousmodifications are possible. For example, the configuration andconnection of certain elements in various embodiments have beendescribed above in the context of a phase-locked loop. In otherembodiments, different configurations can be provided in view of adesign of type of circuit that is to receive temperature, aging, orother compensation.

These and other modifications can be made in light of the above detaileddescription. The terms used in the following claims should not beconstrued to be limited to the specific embodiments disclosed in thespecification.

What is claimed is:
 1. An apparatus, comprising: a first capacitorarray; a sensing logic coupled to the first capacitor array andconfigured to detect a drift in a first control signal and to providethe first capacitor array with a second control signal, wherein thesecond control signal is configured to cause an adjustment ofcapacitance in the first capacitor array based on the detected drift inthe first control signal; and a second capacitor array to receive thefirst control signal and to provide fine frequency tuning in aphase-locked loop.
 2. The apparatus of claim 1, wherein the firstcapacitor array is configured to compensate for temperature or aging ina phase-locked loop.
 3. The apparatus of claim 1, further comprising: athird capacitor array coupled to receive a third control signal toenable or disable capacitors in the third capacitor array to providecoarse frequency tuning.
 4. The apparatus of claim 1, wherein the secondcontrol signal is configured to cause the adjustment of capacitance inthe first capacitor array by enabling or disabling capacitors in thefirst capacitor array.
 5. The apparatus of claim 1, wherein the sensinglogic is configured to detect the drift in the first control signalbased at least in part on a deviation from a threshold.
 6. The apparatusof claim 1, further comprising: a digital loop filter logic coupled toprovide a phase difference to the sensing logic.
 7. The apparatus ofclaim 1, wherein the sensing logic is included in a digital loop filter.8. The apparatus of claim 1, wherein the first capacitor array includesa plurality of fine-tuning capacitor arrays identical to a secondcapacitor array coupled to the sensing logic.
 9. A method, comprising:detecting by a detection circuit, a drift in a first control word; basedat least in part on the detected drift, adjusting by the detectioncircuit, a second control word to be provided to a first capacitor arrayto cause an adjustment in capacitance based on the detected drift in thefirst control word; and providing the first control word to a secondcapacitor array to cause fine frequency tuning in the second capacitorarray.
 10. The method of claim 9, wherein the adjustment in capacitanceis configured to re-center a frequency of a digitally controlledoscillator that includes the first capacitor array.
 11. The method ofclaim 9, further comprising: receiving, by the detection circuit, adigital output signal from a digital loop filter logic.
 12. The methodof claim 9, wherein the detecting and adjusting occur in an all-digitalphase locked loop.
 13. The method of claim 9, wherein the detecting thedrift in the first control word includes detecting a frequency deviationfrom a center of a fine-tuning curve.
 14. The method of claim 9, whereinthe detecting the drift in the first control word includes comparing thefirst control word to a threshold control value.
 15. A system,comprising: a processor; a communication interface coupled to theprocessor, the communication interface including: a first capacitorarray; a second capacitor array; and a sensing logic coupled to thefirst capacitor array and the second capacitor array; and a memoryincluding a non-transitory computer readable medium having instructionsexecutable by the processor to cause the communication interface to:detect by the sensing logic a drift in a first control word; and basedat least in part on the detected drift, adjust, a second control word tobe provided to the first capacitor array to cause an adjustment incapacitance based on the detected drift in the first control word. 16.The system of claim 15, wherein the first capacitor array, the secondcapacitor array, and the sensing logic are included in a digitallycontrolled oscillator.
 17. The system of claim 16, further comprising: athird capacitor array in the digitally controlled oscillator to providecoarse frequency tuning.
 18. The system of claim 17, wherein the thirdcapacitor array is configured to provide the coarse frequency tuning bycancelling process-voltage-temperature variation.
 19. The system ofclaim 15, further comprising: a digital loop filter logic coupled toprovide a signal including a phase difference to the sensing logic. 20.The system of claim 15, wherein the first capacitor array, the secondcapacitor array, and the sensing logic are included in an all-digitalphase-locked loop.